The current expounders of Islamism and Islamic terrorism, mostly Saudi Arabia, Syria, and Iran. Iran is on the edge of overthrowing the mullahs, so I would wait and give some discrete aid to the rebels. Syria and Saudi would not last 48 hours against the forces currently positioned in Iraq. India would take out Pakistan quickly if we ever gave them the green light.
A few more 9/11-type incidents, or even one super-9/11, and it would not be hard to get the American people behind doing a Carthage on the Muslim world. And the US military could take out the combined militaries of the entire remainder of the billion-person Muslim world in 72 hours